EMI Encounters: Separate Your Outhouse & Well

We in the EMC/EMI world often use the saying "Don't put the well next to the outhouse" to describe large EMI sources located next to very sensitive victims.

We in the EMC/EMI world often use the saying "Don't put the well next to the outhouse" to describe large EMI sources (outhouses) located next to very sensitive victims (wells).

Incidentally, this saying probably comes from growing up in Nebraska. I still remember outhouses in my small town. I guess the image stuck.

Here are two of my favorite examples of investigations that involved wells and outhouses.

The first case involved a GPS receiver that was jammed by digital electronics. Although a previous design using similar components had worked fine, the onboard GPS receiver in the new design could not lock on to the satellites.

It was not an easy problem to troubleshoot, since the GPS signal levels are so small in this type of application. The raw GPS signals are in the noise, and sophisticated software techniques are used to recover them. Thus, indirect troubleshooting methods were needed, rather than direct measurements.

Upon examination, I noted the coax cable from the GPS antenna was lying on top of the system processor. I suggested moving it. My client balked (at which point I knew that was not going to be an easy fix), arguing that coax is shielded and doesn't leak.

I countered that no coax is perfect, and that the GPS receiver was looking for nano-Volt signals -- orders of magnitudes below what it would take to jam a nearby digital input.

In any event, when we finally moved the cable, the GPS worked fine. We simply separated the well and the outhouse by a few inches. As a plus, my client no longer believed in the "perfect coax." We've seen similar problems with onboard WiFi and cellular receivers. If you forget everything else, remember this: Radio receivers are extremely sensitive wells.

The second case involved analog sensors that were jammed by a welding arc. A prototype-manufacturing welder used 15 kW of RF to start the arc, which itself could be several inches long. As soon as it started, the electronic controls shut down.

Upon examination, I noted several analog position sensors located within inches of the arc. The sensors, although shielded, were grounded only at one end. This is the preferred technique for 60Hz ground loops, but not for high levels of RF energy.

To make matters worse, the open end of the cable shields were at the sensors.

So we fabricated shield terminations using aluminum foil and hose clamps, and we tried again. Shielding theory said this should work, and it did. But it still amazes me, given the RF power levels. This welder had to be the biggest EMI outhouse I've ever encountered.

The bottom line: Don't put very large EMI sources next to very sensitive EMI victims. Keep the well away from the outhouse.

Daryl Gerke (PE) is a partner at Kimmel Gerke Associates Ltd., an EMI/EMC consulting and training firm. He and his business partner Bill Kimmel (PE) have solved or prevented hundreds of EMI/EMC/ESD problems across a wide range of industries -- computers, industrial controls, medical devices, military systems, vehicles, telecomm, facilities, and more. They have also trained more than 10,000 students in their public and in-house EMC classes. Gerke has a BSEE from the University of Nebraska and resides in Mesa, Ariz.

Even if the coaxial cable is perfect, something that does not get perfect most of the time is "grounding"; An improper grounding would always give rise to ineffective shielding even if a perfect coaxial cable or any other shielded cables are used.

@zeeglen: True!! The solid shield cables provides better performance over the braided shield cables at higher frequencies. At higher frequencies (smaller wavelengths) the braided shield does not provide effective shielding. Is it because the braided shield does not remain as a "low impedance" return path to ground for the high frequency signals? Also may be, because of smaller wavelength of the high frequency signal the braided shield becomes like a perforated shield for high frequency EM radiations and hence ineffective to contain all EM radiations? Or both?

Agree with both of you! Even the best shield (coax or general cable shield) can be rendered ineffective at high frequencies by poor grounding/terminations. As an analogy, even the best garden hose can still leak at the faucets.

As I am fond of saying, cable terminations are such a common EMI problem that I put two kids through college repeatedly solving this problem!

I had a graphic illustration of this a few years ago. I was troubleshooting a customer's Sat TV installation. The dish (LNB) was connected to the receiver by about 30m of coax. The usual coax for this has a braid plus a foil shield, Water had got into the cable and, though the braid was continuous (a bit higher resistance than I would have liked though) the foil had been corroded into nothing. Replaced the cable and sealed it well at the LNB and all was well again. LNBs send to the Rx at around 1 GHz - and the power for the LNB goes the other way - so the cable needs to be good.

@Crusty1: Yes, you are correct...I was trying to understand the technical reason why a solid shield cable would be better than a braided cable; the first thought that occurred to me is that for high frequency signal a poor quality braided cable would be a "leaky feeder"...would give rise to emission issues....what is your opinion on this?
@Daryl: Yes, together we will learn...and this is such a subject, one could continue to learn life long...or as you have correctly said in your other article - we would learn as we get more "battle scars" (failures) in EMC...I liked that phrase you used "battle scar"...very well said...it has become one of my favorites. :-)

@Sanjib.A : I tested a lot of leakeay feeders for use in underground tunnels (London Underground Tube), the shield was a continious copper sheath with repeating slots cut at the correct length for the RF frequency that would be transmitted and recieved by it, some even had multifrequency slots of different length.

So a coax or shielded cable, with a very loose weave, is in my opinion about as good as a tea strainer is for holding water and for the same reason. I always used a continious shield on instrumentation cable for this reason.

I have one bit of information to impart concerning installing screening, be aware that the Electrical installation engineer (for power) has a different idea about screeening(earthing) to that of the instrumentation engineer. I found this out after a lot of field work tracking down noise in systems that had been installed by electrical contractors.

@Crusty1 , Agree with your point on electrical contractors, VFD's qualify as big outhouses! I always keep some 40mm E-cores in my kit so I can make CMT's on sensitive wiring (like encoders). Then I get them to put in equipotential conductors, while I rip up and reroute the signal wires and fix the grounding.

Apart from the leaky-feeder tea-strainer coupling mode which is basically electrostatic in origin, you have an additionally coupling mode due to the non-zero resistance of the shield. Assume a modest 100mohm for a length of coax, then 1 amp of RF coupled into the shield generates 100mV of potential that gets capacitively coupled to the centre conductor and adds 100mV of noise. That's +100dBuV compared to the GPS -30dBuV. So you really need to knock down the RF shield current to a miniscule 1uA.

How do you get RF current in the shield? Easy: make a short circuited loop by grounding both ends of a short piece of coax, and put a changing magnetic field inside the loop. Even if the ends are not physically connected, the capacitance between the GPS antenna groundplane and the chassis will close the loop at 1600MHz.

Apart from cooling everything to liquid helium temperatures, you can manage this mode by

adding thick equipotential ground cables, so the noise currents flow through the path of least resistance, NOT your shield

increasing the inductance of the coax (ferrite beads, clamshell ferrites or Common Mode Transformer (My favorite))

use a double screen cable or Triax so that noise current doesn't flow through the inner shield

judicious cable routing to avoid doing a lap around the outhouse

generous groundplanes and power planes to minimise the magnetic field generating volume

Use differential signalling if possible

Just because a conductor is at ground potential doesn't mean it can't radiate! Circulating ground currents easily couple into adjacent signal conductors.